Cardiac events, such as heart attack, stroke and sudden cardiac death occur with increased frequency in the morning hours. In these studies we examined the importance of activation of platelets to this phenomenon. Using platelet aggregation studies, we demonstrated increased platelet activation during arousal and exercise. However, this was not associated with changes in the receptors on the surface of platelets. The increased aggregation of platelets was probably secondary to increases in platelet count, fibrinogen levels and circulating catecholamine levels. In our second study we examined whether blocking the effects of catecholamines on the platelet surface would reduce the morning increase in platelet activation. Yohimbine, an agent that blocks alpha-2 receptors on the platelet surface, was given orally, and was shown to inhibit the increase in platelet aggregation that occurred with arousal in the early morning. However, the increasing platelet aggregation associated with exercise remained unaffected by Yohimbine. These studies therefore demonstrate that the morning increase in platelet aggregation due to arousal and assumption of upright posture is, at least partly, contributed to by the increase in levels of circulating catecholamines. Increase in platelet aggregability may be an important physiologic event that predisposes to the higher incidence of cardiac and vascular events in the morning hours.